Many applications require small, light weight, efficient conformal antennas. Traditionally, microstrip patch antennas have been a preferred type for many applications. These applications tend to be only over a narrow frequency band, since microstrip patch antennas typically are efficient only in a narrow frequency band. Otherwise, the advantages of these antennas of being mountable in a small space, of having high gain and of being capable of being constructed in a rugged form, have made them the antennas of choice in many applications.
Satellite communication (Satcom) systems and other similar communications systems require relatively broadband antennas. Typical military broadband applications include long range communication links for smart weapon targeting and real time mission planning and reporting. A variety of antenna designs, such as crossed slots, spirals, cavity-backed turnstiles, and dipole/monopole hybrids have been used for similar applications over at least the last 15 years. However, most of these antennas require large installation footprints, typically for UHF antennas, a square which is two to three feet on a side. When used on aircraft, these antennas intrude into the aircraft by as much as 12" and can protrude into the airstream as much as 14". For airborne Satcom applications, antennas of this size are unacceptably large, especially on smaller aircraft, and difficult to hide on larger aircraft, where it is undesirable to advertise the presence of a UHF Satcom capability. Therefore, there has been a need for a small highly efficient broadband antennas.
As illustrated in FIG. 29, further problems arise when attempting to couple the feeds 406 and 408 of a microstrip antenna 404 to a power generator 402, especially in high power applications. It is generally desirable to present a power generator with a good load match, i.e. VSWR, at all limes. That is, it is generally desirable to minimize the amount of reflected power P.sub.r, such as that due to antenna impedance mismatch, that is reflected back from the antenna to the power generator. Moreover, it is sometimes necessary to feed the antenna with a phase shift.
Typically, a 90.degree. hybrid coupler 400 such as that illustrated in FIG. 29 will provide the desired phase while allowing the power reflected back from the antenna 404 to be absorbed and dissipated by a termination 410. The amount of power that can be dissipated by the termination, however, is dependent on the physical size of the termination. The dissipated power creates excess heat which can burn out the termination if it is too small. In high power applications, this means that a physically large termination is required to absorb the reflected power caused by antenna mismatch. Moreover, the termination must be located away from the antenna assembly. Therefore, there has also been a need in the art for a feed network that can effectively present a good load match between a power generator and a microstrip antenna, but that does not occupy a large amount of space.